1. Field of the Invention
The present invention relates to an image forming apparatus that performs light exposure using a light source that has a plurality of light-emitting portions.
2. Description of the Related Art
In general, an image forming apparatus according to an electrophotographic system scans a photosensitive member by reflecting a beam that is emitted from a laser diode using a polygonal mirror that is rotationally driven. Each plane of the polygonal mirror is a mirror, and laser lights are respectively reflected on each plane to draw lines (scanning lines) on the photosensitive member. At this time, if the tilt of each plane with respect to the rotational axis is equal, the line pitch of lines drawn by each plane will be constant.
However, due to variations in the accuracy of finishing, the tilt of each plane with respect to the rotational axis do not match exactly. Consequently, variations arise in the line pitches drawn by each plane in a cycle according to the number of planes of the polygonal mirror. As a result, for example, even when using image data which comprises a plurality of equidistant horizontal lines, the pitch of each horizontal line in the formed image will not match.
Technology which corrects this kind of pitch nonuniformity in the sub-scanning direction has already been proposed. Japanese Patent Laid-Open No. 2005-234510 proposes technology which, by varying the light power rate between a principal light source and a secondary light source corresponding to two adjacent lines, corrects pixel error in the sub-scanning direction with a high degree of accuracy.
Japanese Patent Laid-Open No. 63-050809, Japanese Patent Laid-Open No. 5-208523 and Japanese Patent Laid-Open No. 9-159948 also propose technologies that change an optical path in a direction which cancels a plane tilt using a prism that is provided between the end face of a laser and a polygonal mirror.
However, according to the technology disclosed in Japanese Patent Laid-Open No. 2005-234510, a spot produced by a beam from the primary light source and a spot produced by a beam from the secondary light source do not match on the photosensitive member. More specifically, the shape of the spots is that of FIG. 8. Therefore, the size of a dot that is formed by the technology disclosed in Japanese Patent Laid-Open No. 2005-234510 is larger than the size of a dot formed by a single light source. Hence, according to the technology disclosed in Japanese Patent Laid-Open No. 2005-234510, it is difficult to form a high definition image or to enhance the reproducibility of dots one by one to form a sharper latent image.
Further, the respective technologies disclosed in Japanese Patent Laid-Open No. 63-050809, Japanese Patent Laid-Open No. 5-208523 and Japanese Patent Laid-Open No. 9-159948 require an additional optical component such as a prism and a drive circuit to accompany that optical component, and therefore production costs are liable to rise and the adjustment time at assembly is also liable to increase.